Add scheme build architecture overrides (#1642)

* Add scheme architecture override tests

* Add scheme build architecture overrides

* Test scheme architecture JSON encoding

* Document scheme build architecture overrides

* Update changelog for #1642

* Default scheme buildArchitectures to matchRunDestination

- Change the default to matchRunDestination, matching Xcode's default for new schemes (Xcode 15.3+). Use useTargetSettings to keep the previous behaviour.
- Add buildArchitectures to target schemes, so schemes generated from a target's `scheme` can override it too.
- Document the options with their Xcode scheme editor names.
- Move the changelog entries into Next Version and note the new default.
- Regenerate fixtures.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* Restore TestProject fixture

The previous commit accidentally included a TestProject project.pbxproj regenerated by a local test run, which dropped the synced folder exception set.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Co-authored-by: Yonas Kolb <yonaskolb@users.noreply.github.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>